(1) Field of the Invention
The present invention relates to a sealed battery used as a power supply for portable electronic devices and the like.
(2) Description of the Related Art
The rapid spread of portable electronic devices in recent years has brought about the wide spread of sealed batteries as power supplies for those devices. As electronic devices have achieved better performance and compactness, there are also demands that sealed batteries become miniaturized, lightweight, and high-energy.
Among sealed batteries, sealed secondary batteries, which is rechargeable for repeated use, have been widely and remarkably spread these days, and especially, non-aqueous secondary batteries such as lithium ion batteries have come into wide use for their advantages in light weight and high energy density.
The following explains the structure of a sealed battery of the prior art, with a cylindrical lithium ion battery taken as an example.
A cylindrical lithium ion battery has a spiral-wound electrode assembly therein, which was obtained by spirally winding a structure made up of a positive sheet, a negative sheet, and a separator interposed therebetween, the spiral-wound electrode assembly being cased in a can with electrolyte being charged therein, and the can being sealed with a sealing lid.
Generally speaking, the spiral-wound electrode assembly is disposed around a space, which is the result of having removed a core used for spirally winding the electrode assembly. In some cases, a center pin that is cylindrical and tube-shaped is inserted in this space. The center pin is provided for the following two purposes:
The first purpose is to suppress deformation of the sheets in the spiral-wound electrode assembly caused by repeated recharging and discharging. More specifically, as a lithium ion battery repeatedly goes through recharging and discharging, the spiral-wound electrode assembly will expand, and since it cannot expand outward because of the can, it expands mainly toward the center space. At this time, the sheets in the spiral-wound electrode assembly may get deformed near the center space, and a short circuit may occur in the deformed part. Thus, the center pin is inserted to prevent the spiral-wound electrode assembly from being deformed toward the center space, and the sheets from being deformed near the center space.
The second purpose is to prevent the lithium ion battery from rupturing when the lithium ion battery is thrown into a fire by mistake and the temperature inside gets over 200 degrees Celsius, with the cylindrical and tube-shaped center pin acting as a gas exhaust duct inside the battery. Usually, a separator in a lithium ion battery is made of a microporous membrane, for example a polyethylene membrane, and such a separator will melt at a temperature of 200 degrees Celsius or higher. In such a case, the melted separator will flow into the center space that is acting as a gas exhaust duct, and obstruct the passage. Thus, the center pin is inserted to keep the gas exhaust duct open at the center of the spiral-wound electrode assembly even if the separator melts in the event that the battery is thrown into a fire.
The center pin, serving the above-mentioned purposes, is normally formed by rolling up a metal thinplate into a cylindrical tube shape for reasons related to the cost and the gas exhaust efficiency, and the two edges of the plate in the circumferential direction are usually not joined together; therefore, there remains a slit between the two edges of the plate.
When a lithium ion battery gets dropped from a height, or conversely, something gets dropped onto a lithium ion battery from a height, the can may be also deformed. When such an external force is applied to a lithium ion battery as to deform the can, the center pin inside may be also deformed. In the event that the center pin gets deformed to a large extent, the separator may be damaged, or an edge of the slit of the center pin may thrust into the spiral-wound electrode assembly.
In case the separator gets damaged, a short circuit will occur inside the lithium ion battery; however, since the concerning area is relatively large and the short-circuit current density is small, the battery will not have such a high temperature as approximately 130 degrees Celsius or higher.
On the other hand, in case the separator is not damaged, but an edge of the slit of the center pin has thrust into the spiral-wound electrode assembly, the concerning area of the short circuit inside the spiral-wound electrode assembly is small and the short-circuit current density is large due to a local short circuit; the battery therefore is likely to have a very high temperature of approximately 130 degrees Celsius or higher.